In injection molding of thermoplastic materials, when such materials are fed from the injection nozzle of an injection molding machine into a cavity mold, the materials pass through a number of channels before finally flowing into the cavity in which the desired piece to be molded is formed. The molding material located in the channel leading from the injection machine into the mold is usually called a "sprue" while the materials in the channels within the mold itself are usually referred to as "runners". It is often desirable to prevent solidification of the sprue or the runners, or both, between successive injections in order to avoid waste of the sprue and runner material and otherwise to improve the speed and the quality of the operation of the system. For this purpose heated bushings have been used to contain the sprue and sometimes to contain the runners. Heaters have been located within the bushing bore and have been heated by electrical heating elements embedded within the heater. Examples of such sprue heaters are shown in U.S. Pat. Nos. 3,010,155 and 3,767,340. However, prior art devices suffer from several weaknesses. Due to the nature of the heater it is difficult to maintain a uniform temperature throughout the sprue. Hot spots and cool spots occur which produce nonuniform heating of the sprue material which results in a non-uniform melt being injected into the mold. Also the introduction of a heater into the sprue bore unduly increases the surfaces to which the molten material of the sprue may adhere and contaminate a suceeding injection. This is particularly objectionable in the case where the suceeding injection is of a different color from the proceeding injection. The presence of the heater within the sprue or runner channel also raises the pressure drop in the flow of the molten material and the resistance to such flow. Also such heaters involve an undesirable degree of maintainance, requiring frequent replacement of the heating element with consequent interruptions in the operation of the equipment.